1. Field of the Invention
The invention relates to a system for checking the quality of a digital connection made by means of a telephone circuit.
It is known to check the quality of a digital connection by transmitting a check message at one end of a telephone circuit using this connection and by comparing the message received at the other end with the check message and counting the errors. The same operations are repeated in the opposite direction after the connection is made.
2. Description of the Prior Art
French Patent Specification No. 2,130,063 filed on Dec. 10, 1971, by the so-called ALITALIA Company describes a device for making a check of the mentioned kind. FIG. 1 of the said Specification, which is also used as FIG. 1 of the present Application, shows a digital link circuit M between ends A and B, comprising a transmission line and two modems (modulatordemodulators).
In order to check the quality of the connection in FIG. 1 two identical devices 1 and 2 each comprising a transmitter STT.sub.1 or STT.sub.2 and a receiver TR.sub.1 or TR.sub.2 are connected to the ends A and B of a telephone circuit. The devices are put into operation by an operator, e.g. operating at end A, who causes the check message to be transmitted. The device at end B compares the received message, bit by bit, with the simultaneously-produced check message. The first error results in a stoppage in the comparison in progress in the device at end B of the circuit, so that the operator can note the error. Next, the operator restarts the device. Correct transmission of the check message in one direction causes transmission of the same message in the opposite direction, and so on.
In the event of transmission errors, however, the devices described in the above cited patent require the operator to make certain that the error is systematic. Such an operation is slow, since it requires manual intervention. Stoppage of the device as soon as a received bit is incorrect entails the necessity that a signal returning to the beginning of the message, for the purpose of resetting the device, be transmitted from the end of the circuit where the check was made to the end where the check message was transmitted.
The invention relates to a system for checking the quality of a digital connection link whereby the quality can be ascertained at each end of the telephone circuit constituting the said link.